Wellbore Cooling as a Means To Permanently Increase Fracture Gradient

摘要

In recent years, several techniques have been proposed toincrease the fracture gradient by inducing changes in the nearwellbore region (Alberty and McLean, 2004; Sweatman, et al.,2004; Benaissa et al., 2005). This process, often called“wellbore strengthening”, has most recently been implementedby adding specially designed proppant material to the mudbefore raising its pressure above the fracture gradient. Thegoal was to induce short tensile fractures in the vicinity of thewellbore wall which are prevented from propagating; thus,creating a “stress cage”. However, this method has oftenproven ineffective in low permeability formations wheremainly uncontrolled fracture propagation occurs.The purpose of this paper is to propose and evaluate the use ofwellbore cooling, in combination with more classicalstengthening processes, to permanently increase the fracturegradient without the risk of circulation losses inherent in the“stress cage” method, as it is currently applied. This approachinvolves lowering the temperature of the drilling mud; thus,reducing the hoop stress at the borehole wall and then ‘setting’the stress cage in the standard manner. Tensile cracks canthen be induced at significant lower mud weights. Given thetypical thermal conductivity properties of rocks, the tensilestresses induced by cooling (and consequently, the createdfractures) will tend to be confined to the near wellbore region.This work presents an evaluation of the effect cooling has onthe stress profile of a “solid” material and compares it with afully coupled thermoporoelastic solution. The results of suchanalyses may then be used to design a field application to testthis novel idea.